Optical sensing approaches for biological and chemical measurements have generally relied on fluorescence and surface plasmon resonance (SPR) techniques. Non-optical based sensors are far less sensitive than SPR sensors, and employ radiolabeled and fluorescently labeled proteins in immunoassay formats. The reduced sensitivity of the non-optical sensors creates measurement inaccuracies and requires more tightly-controlled testing environments.
Contemporary bulk SPR-based techniques are difficult to transition into a mobile platform because critical component alignments cannot be maintained in a field environment. Impractical size and weight ramifications are necessary to "ruggedize" current systems. Attempts to solve this dilemma have resulted in the creation of portable sensing systems through the construction of optical fiber versions of both SPR and optical fluorescence systems. These devices, however, are severely limited. In both the "ruggedized" and the portable sensing scenarios, time-consuming and expensive manufacturing processes are required.
Jorgenson et al., (R. C. Jorgenson and S. S. Yee, "A Fiber-Optic Chemical Sensor Based On Surface Plasmon Resonance", Sensors and Actuators B, volume 12, pages 213-220, 1993), demonstrated an SPR approach in a fiber optic configuration. Their approach involved a fiber-optic SPR sensing configuration that did not require a light-coupling prism. They disclose the sensing element as a segment of fiber in which the cladding has been removed and a 550 .ANG. thick silver film has been symmetrically deposited on the fiber core, via electron-beam evaporation. Removal of the cladding enables the light to interact with the metallic layer.
Frances Ligler et al., (Frances S. Ligler, Lisa C. Shriver-Lake, Joel P.
Golden, and George P. Anderson, "The Antibody-Based Biosensor: A Technology For Today", Naval Research Reviews, vol. 46, 3: 13-17, 1994) employed long optical fibers as a detection means that could easily be manipulated into various sample containers. They removed the plastic cladding to fabricate the sensing region before antibodies were attached and the fiber immersed in the sample. As fluorescent light returns up the fiber, it passes from the unclad region of the fiber into the clad region of the fiber. The antibodies were used to capture dye-marked targets, with the returning fluorescent signal indicating target strength. It was found, however, that much of the emitted light was lost into the cladding, thus necessitating changing the geometry of the fiber. Ligler et al. tapered the fiber to enhance both the distribution of the excitation light throughout the length of the probe region and the preservation of the returning fluorescent signal. This probe uses light propagating in the tapered region of the optical fiber to produce an evanescent field extending into the coating.
Anderson et al., (George P. Anderson, Joel P. Golden, Lynn K. Cao, Daya Wijesuriya, Lisa C. Shriver-Lake, and Francis S. Ligler, "Development of an Evanescent Wave Fiber Optic Biosensor", IEEE Engineering in Medicine and Biology, 358-363, June/July 1994), disclosed a fiber optic biosensor where several centimeters of cladding area are removed along the fiber's distal end. Recognition antibodies are immobilized on the exposed core surface. These antibodies bind fluorophore-antigen complexes within the interaction region of the evanescent wave. The evanescent wave effectively penetrates less than a wavelength beyond the core into the surrounding medium and is what excites the fluorescent molecules in complexes bound to the surface of the waveguide core. The resulting fluorescence couples into the cladded core of the immersed optical fiber. This immersed probe functions as a dielectric waveguide with the aqueous buffer as the cladding medium. The fluorescent signal generated within the probe is then transmitted to the fluorimeter through the clad fiber. They further disclosed that the primary difficulty in utilizing the core surface as the sensing region is that removing the cladding causes an abrupt disturbance in the dielectric structure of the optical fiber. Because the immersed core supports more modes than the clad fiber, some of the fluorescent signal is lost upon entry into the clad fiber.
Compounding this difficulty, fluorescence coupling from outside the core propagates primarily in the highest-order modes available, thus exacerbating signal loss from V-number mismatch. To solve this problem, Anderson et al. provided a combination tapered probe by removing the buffer and most of the cladding from the distal end of the fiber. This created a probe that tapered from the original radius rapidly down to a V-number matching radius, while maintaining total internal reflection. They note that it is critical that the probe be reduced to the V-number matching radius to ensure that fluorescence entering the fiber probe will be captured in modes that also propagate in the clad fiber.
The primary failure of this sensor is that the physical dimensions of the optical fiber must be altered to effectively excite the sensing mechanism. Etching techniques usually involve exposing an optical fiber to a chemical bath for a set period, followed by a stop bath, thus etching away a significant portion of the cladding and producing a brittle structure through the removal of protective buffer coatings, reduction of fiber diameter, adverse handling of exposed unprotected fiber, and increased probability of microcracks. Furthermore, the process of precise uniform etching of optical fibers is tedious, time-consuming, involves hazardous materials, and is not cost-effective for mass produced devices.
Tapering of an optical fiber creates similar problems. One method of tapering an optical fiber involves securing the fiber at two discrete locations. The fiber is then exposed to a localized high temperature perturbation, such as a propane torch, to soften the optical fiber. As the glass structure melts, the optical fiber is simultaneously pulled by opposing forces, resulting in a tapered reduction of the fiber diameter.
Etching and tapering of optical fibers is difficult to control and results in a weakened sensing element. Prior methods have required these steps to either excite an SPR wave in a metallic layer on the fiber or create an evanescent wave to excite captured targets dyed with fluorescent material. The present invention, does not require etching or tapering of an optical fiber. Rather, it utilizes a long period grating to excite confined propagating broadband light into higher order modes which causes an excitation of a sensing mechanism such as an SPR wave or a bound fluorescent material. This yields tremendous advantages because long period gratings may be produced cost-effectively, in large volumes, and without causing any degradation in fiber strength.
An object of the present invention is to provide an optical sensor that does not require etching or tapering of the waveguide.
Another object of the present invention is to provide an optical sensor having at least one long period grating that promotes an excitation of confined propagating broadband light into higher order modes which causes an excitation of a sensing mechanism.
Another object of the present invention is to provide an optical sensor having at least one pair of long period gratings disposed within an optical waveguide where the first long period grating promotes excitation of broadband light into higher order modes causing an excitation of a sensing mechanism where the second long period grating then directs light back into the optical waveguide.